Abstract: In February 1991, the authors collected ambient vibration measurements to study the behavior of four flat plate, reinforced concrete structures on Manhattan island in New York City. The buildings range from 27 to 52 stories and all but one structure rest on a rock supported foundation. This report presents the development of ambient vibration analysis software based on the fast Fourier transform. The results provide a base-line set of periods, deflections, and damping ratios for comparison with results expected as a result of strong winds. Periods estimated from ambient, most wind-induced vibrations are used to calibrate finite element models of the structures and are compared with values calculated from code design rules.